The present invention relates to a method of manufacturing an aluminum alloy plate for a lithographic printing plate. The invention also relates to an aluminum alloy plate for a lithographic printing plate obtained by the manufacturing method described above, a lithographic printing plate support, and a presensitized plate.
The method of manufacturing aluminum alloy plate for a lithographic printing plate by continuous casting, more specifically the manufacturing method of aluminum alloy plate for a lithographic printing plate that includes a casting step involving melting an aluminum starting material to obtain an aluminum melt, filtering the aluminum melt, feeding the filtered aluminum melt through a melt feed nozzle between a pair of cooling rollers and rolling it while solidifying between the pair of cooling rollers thereby forming an aluminum alloy plate, and also includes a cold rolling step, an intermediate annealing step, a finish cold rolling step and a flatness correcting step and in which an aluminum alloy plate with a thickness of 0.1 to 0.5 mm is obtained is simple in its steps and has therefore such advantages as reduced losses, higher yield, insusceptibility to step variations, lower initial equipment cost, and lower running cost compared to a conventional method of manufacturing aluminum alloy plate for a lithographic printing plate support that includes a direct chill casting step, a scalping step, a soaking step, a heating step and a hot rolling step.
An aluminum alloy plate for use in a lithographic printing plate is generally employed after its surface has been roughened for the purpose of controlling the adhesion to the image forming layer and the balance between water retention and ink retention. In such surface roughening treatment, the surface properties of the aluminum alloy plate play an extremely important role. Important factors that may determine the properties in surface roughening treatment include not only the conditions in surface roughening treatment but also the surface composition of the aluminum alloy plate and its uniformity. Nonuniform surface composition may lead to nonuniform treatment, thus causing surface defects such as surface unevenness. It is necessary to develop a method of producing an aluminum alloy plate having a uniform surface composition in order to solve this problem.
Exemplary methods of improving the surface uniformity that are known in the art include a method in which a nozzle with a high thermal conductivity is used to enhance the uniformity in the solidification in a cross-sectional width direction (see JP 2006-15361 A), and a method in which the angle formed between the normal from the center of a roll and a meniscus formed just upstream of the roll is set within a certain range to suppress periodic variations in cooling rate (see JP 2006-130545 A).
In another respect, it is known to control the height of the melt level in order to stabilize casting. Exemplary known methods that may be used to stabilize the liquid level include a method in which a non-contact type or contact type liquid level sensor of an electromagnetic induction system is used to continuously detect the liquid level and feedback is given to a mechanism for controlling the melt flow (inflow) rate (e.g., a movable valve for preventing the melt from flowing) (see JP 3781211 B), and a method in which a dummy volume which is movable in the vertical direction is inserted and vertically moved in accordance with changes in the amount of molten metal to stabilize the melt level height (see JP 8-238541 A). These methods may be used to stabilize the liquid level height in at least several seconds or several tens of seconds.
However, these methods are all not sufficient for improving the surface defects.
In still another respect, JP 7-132689 A describes that some portions of an aluminum surface having an iron concentration of at least 1% account for 0.01 to 10% of the entire surface area. This prior art invention aims at improving the nonuniform shape of the pits formed at the surface by electrolytic graining treatment. Since the area to be analyzed for the distribution of iron as described in JP 7-132689 A is on the order of micrometers and is therefore extremely narrow, an effect of improving the uniformity in electrolytic etching during electrolytic graining treatment can be expected, but segregation over a visually noticeable range as wide as several millimeters to several tens of centimeters and surface defects cannot be improved, which indicates that such surface defects are not sufficiently improved.